Corrosion Resistant Barrier Layer for a Solid Oxide Fuel Cell Stack and Method of Making Thereof

ABSTRACT

A method of forming diffusion barrier layer includes providing an interconnect for a fuel cell stack, forming a glass barrier precursor layer over a Mn and/or Co containing electrically conductive contact layer on the interconnect, and heating the barrier precursor layer to precipitate crystals in the barrier precursor layer to convert the barrier precursor layer to a glass ceramic barrier layer.

RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/824,025, filed on May 16, 2013, the entire contentsof which are incorporated herein by reference.

FIELD

The present invention is generally directed to solid oxide fuel cells(SOFCs) and more specifically to forming coatings that provide corrosionresistance to electrolytes for solid oxide fuel cells.

BACKGROUND

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy with high efficiencies. Electrolyzer cellsare electrochemical devices which can use electrical energy to reduce agiven material, such as water, to generate a fuel, such as hydrogen. Thefuel and electrolyzer cells may comprise reversible cells which operatein both fuel cell and electrolysis mode.

In a high temperature fuel cell system, such as a solid oxide fuel cell(SOFC) system, an oxidizing flow is passed through the cathode side ofthe fuel cell, while a fuel flow is passed through the anode side of thefuel cell. The oxidizing flow is typically air, while the fuel flow canbe a hydrocarbon fuel, such as methane, natural gas, propane, ethanol,or methanol. The fuel cell, operating at a typical temperature between750° C. and 950° C., enables combination of the oxygen and freehydrogen, leaving surplus electrons behind. The excess electrons arerouted back to the cathode side of the fuel cell through an electricalcircuit completed between anode and cathode, resulting in an electricalcurrent flow through the circuit.

Fuel cell stacks may be either internally or externally manifolded forfuel and air. In internally manifolded stacks, the fuel and air isdistributed to each cell using risers contained within the stack. Inother words, the gas flows through openings or holes in the supportinglayer of each fuel cell, such as the electrolyte layer, and gasseparator of each cell. In externally manifolded stacks, the stack isopen on the fuel and air inlet and outlet sides, and the fuel and airare introduced and collected independently of the stack hardware. Forexample, the inlet and outlet fuel and air flow in separate channelsbetween the stack and the manifold housing in which the stack islocated.

Fuel cell stacks are frequently built from a multiplicity of cells inthe form of planar elements, tubes, or other geometries. Fuel cellstacks, particularly those with planar geometry, often use seals betweenelectrolyte and interconnect surfaces to contain fuel and air at variouslocations within the stack. As shown in FIG. 1, in fuel cell stacks thatare internally manifolded for fuel (i.e., in which fuel is providedthrough fuel riser openings in SOFCs and interconnects in the stack)electrolyte crack formation has been observed at ring seals initiated bycell electrolyte corrosion. A ring seal is a seal that surrounds thefuel inlet and fuel outlet riser openings between the cathode (i.e.,air) side of a given SOFC and an air side of an adjacent interconnect(also known as a gas separator plate). This corrosion in conjunctionwith stresses which occur during operation lead to cracks, cell crackingand catastrophic failure at elevated temperatures (e.g., after 2 hoursat 900 C) as shown in FIG. 2.

SUMMARY

An embodiment relates to a method of forming a diffusion barrier layer,comprising providing an interconnect for a fuel cell stack, wherein theinterconnect contains an electrically conductive contact layer locatedon an air surface of the interconnect, and wherein the electricallyconductive contact layer contains at least one of Co and Mn, forming abarrier precursor layer comprising at least 90 wt. % glass over theelectrically conductive contact layer, and heating the barrier precursorlayer to precipitate crystals in the barrier precursor layer to convertthe barrier precursor layer to a glass ceramic barrier layer.

Another embodiment relates to a solid oxide fuel cell (SOFC) stack,comprising a plurality of SOFCs and a plurality of interconnects. Eachof the plurality of the interconnects is located between two adjacentSOFCs. Each of the plurality of the interconnect comprises anelectrically conductive contact layer located on an air surface of theinterconnect, the electrically conductive contact layer containing atleast one of Co and Mn, and a glass ceramic barrier layer located overthe electrically conductive contact layer. The glass ceramic barrierlayer comprises zirconium silicate (ZrSiO₄) crystals, barium aluminumsilicate (BaAlSiO₄) crystals and potassium feldspar (KAlSi₃O₈) crystalslocated in a glassy matrix.

Another embodiment relates to an interconnect for a fuel cell stack,comprising an interconnect body having an air surface having air flowchannels and ribs and a fuel surface having fuel flow channels and ribs,an electrically conductive contact layer located on the air surface ofthe interconnect, the electrically conductive contact layer containingat least one of Co and Mn, and a first layer located over theelectrically conductive contact layer. The first layer comprises:

-   45-55 wt. % silica (SiO₂);-   5-10 wt. % potassium oxide (K₂O);-   2-5 wt. % calcium oxide (CaO);-   2-5 wt. % barium oxide (BaO);-   0-1 wt. % boron trioxide (B₂O₃);-   15-25 wt. % alumina (Al₂O₃); and-   20-30 wt. % zirconia (ZrO₂) on an oxide weight basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are photographs showing cell electrolyte corrosion andcracking in prior art SOFC stacks. FIG. 1 is a close up of the corrosionand FIG. 2 is a top view of a SOFC containing a crack (circled).

FIG. 3 illustrates a side cross sectional view of a SOFC stack alonglines A-A in FIGS. 4A and 5A of an embodiment of the invention.

FIGS. 4A and 4B are top views of an air side of an interconnect of anembodiment of the present invention.

FIG. 4C is a top view of a fuel side of the interconnect.

FIGS. 5A and 5B are top views of an air side of fuel cells of anembodiment of the invention.

FIG. 6 illustrates a portion of a side cross sectional view of a priorart SOFC stack.

FIG. 7 is a schematic illustration of a theory of intergranularcorrosion of the electrolyte of a prior art SOFC stack.

FIGS. 8A-8C illustrate a portion of a side cross sectional view of aSOFC stack along lines B-B in FIGS. 4B and 5A of embodiments of theinvention.

DETAILED DESCRIPTION

The present inventors realized that solid oxide fuel cell electrolytecorrosion and cracking may be reduced or eliminated by reducing oreliminating manganese diffusion from a manganese containing,electrically conductive contact layer on the interconnect into theceramic electrolyte. The inventors have observed that manganese from themanganese containing layer diffuses or leaches into a glass or glassceramic seal and the manganese (and/or a manganese containing compound,such as a manganese rich silicate) then diffuses into the zirconia basedelectrolyte and accumulates at the electrolyte grain boundaries,resulting in intergranular corrosion of the electrolyte. The inventorsfurther observed that absent a glass seal, manganese from the contactlayer located on the interconnect does not attack the zirconia basedelectrolyte, such as yttria and/or scandia stabilized zirconia. In fact,the SOFC cathode electrode directly on the electrolyte may comprise LSMwithout attacking the electrolyte. Thus, in an embodiment in which astack is internally manifolded for fuel, corrosion of the stabilizedzirconia electrolyte can be reduced or prevented by isolating theelectrolyte from manganese diffusion from the conductive contact layerby depositing a manganese diffusion barrier between the manganesecontaining contact layer and the glass seal. In another embodiment, thebarrier layer may be deposited between the glass seal and the stabilizedzirconia electrolyte. Alternatively, barrier layers may be depositedbetween both the manganese containing barrier layer and the glass sealand between the glass seal and the stabilized zirconia electrolyte. Thebarrier layer may be used with any manganese and/or cobalt containingmetal oxide contact layer on the interconnect, such as a perovskitelayer (e.g., lanthanum strontium manganate (“LSM”), lanthanum strontiumcobaltite or lanthanum strontium manganate-cobaltite), a spinel layer(e.g., a manganese cobalt oxide spinel, such as a Mn_(x)Co_(3-x)O₄spinel(“MCO”), where x ranges between 1 and 2) or a mixture of a perovskiteand spinel metal oxide (e.g., a mixed LSM and MCO layer). However, LSMis used as an exemplary metal oxide coating below for brevity.

FIG. 3 illustrates a side cross sectional view through a middle ofplanar solid oxide fuel cell (SOFC) stack 100. The stack comprises aplurality of solid oxide fuel cells 1 and a plurality ofinterconnects/gas separator plates 9. Each cell 1 includes an anodeelectrode 3, a solid oxide electrolyte 5 and a cathode electrode 7. Theanode electrode 3 may comprise a cermet having a metal phase, such as anickel or nickel oxide phase and a ceramic phase, such as a doped ceria(such as samaria or gadolinia doped ceria) and/or a stabilized zirconia,such as yttria or scandia stabilized zirconia. The anode 3 may compriseone or more sublayers comprising the above described cermet or ceramicmaterials. The electrolyte 5 may comprise a stabilized zirconia, such asscandia stabilized zirconia (SSZ) or yttria stabilized zirconia (YSZ).Alternatively, the electrolyte 5 may comprise another ionicallyconductive material, such as a doped ceria. The cathode electrode 7 maycomprise an electrically conductive material, such as an electricallyconductive perovskite material, such as lanthanum strontium manganite(LSM). Other conductive perovskites, such as LSCo, etc., or metals, suchas Pt, may also be used.

FIG. 3 shows that the lower SOFC 1 is located between two interconnects9. Each interconnect/gas flow separator plate 9 separates fuel, such asa hydrogen and/or a hydrocarbon fuel, flowing to the fuel electrode(i.e. anode 3) of one cell in the stack from oxidant, such as air,flowing to the air electrode (i.e. cathode 7) of an adjacent cell in thestack. The interconnect 9 contains gas flow passages or channels 8between the ribs 10. The interconnect electrically connects the fuelelectrode 3 of one cell to the air electrode 7 of the adjacent cell. Theinterconnect is made of or contains electrically conductive material,such as chromium or an alloy thereof, such as chromium-iron,chromium-yttrium, chromium-iron-yttrium, etc. A first electricallyconductive contact layer, such as a metal oxide perovskite and/or spinellayer 11, is provided on the air side of the interconnect 9 (i.e.,between the interconnect and the cathode electrode 7 of an adjacent fuelcell 1). Layer 11 coats the tops of the ribs 10, the sides of the ribs10 (not shown for clarity) and the bottoms of the flow passages 8. Anoptional second electrically conductive contact layer (not shown), suchas a nickel contact layer, may be provided on the fuel side of theinterconnect (i.e., between the anode electrode and the interconnect).

While vertically oriented stacks are shown in FIG. 3, the fuel cells maybe stacked horizontally or in any other suitable direction betweenvertical and horizontal. The term “fuel cell stack,” as used herein,means a plurality of stacked fuel cells which share a common fuel inletand exhaust passages or risers. The “fuel cell stack,” as used herein,includes a distinct electrical entity which contains two end plateswhich are connected to power conditioning equipment and the power (i.e.,electricity) output of the stack. Thus, in some configurations, theelectrical power output from such a distinct electrical entity may beseparately controlled from other stacks. The term “fuel cell stack” asused herein, also includes a part of the distinct electrical entity. Forexample, the stacks may share the same end plates. In this case, thestacks jointly comprise a distinct electrical entity. In this case, theelectrical power output from both stacks cannot be separatelycontrolled.

FIG. 4A shows the air side of an exemplary interconnect 9. Theinterconnect may be used in a stack which is internally manifolded forfuel and externally manifolded for air. The interconnect contains airflow passages or channels 8 between ribs 10 to allow air to flow fromone side 13 to the opposite side 14 of the interconnect. Ring seals 15are located around fuel inlet and outlet openings 16A, 16B (i.e.,through holes 16A, 16B which comprise part of the respective fuel inletand outlet riser openings extending through interconnect 9). Strip seals(not shown) are located on lateral sides of the interconnect 1. FIG. 4Bshows a close up view of an exemplary seal 15, passages 8 and ribs 10.The seals 15 may comprise any suitable seal glass or glass ceramicmaterial, such as borosilicate glass. Alternatively, the seals 15 maycomprise a glass ceramic material described in U.S. application Ser. No.12/292,078 filed on Nov. 12, 2008, incorporated herein by reference.

The interconnect 9 may contain an upraised or boss region below the seal15 if desired. If desired, the interconnect may be configured for astack which is internally manifolded for both air and fuel. In thiscase, the interconnect and the corresponding fuel cell electrolyte wouldalso contain additional air inlet and outlet openings.

FIG. 4C illustrates the fuel side of the interconnect 9. A window seal18 is located on the periphery of the interconnect 9. Also shown arefuel distribution plenums 17 and fuel flow passages 8 between ribs 10.It is important to note that the interconnect shown in FIG. 4C has twotypes of fuel flow passages; however, this is not a limitation of thepresent invention. The fuel side of an interconnect may have fuel flowpassages that are all the same depth and length, or a combination ofshort and long, and/or deep and shallow passages.

FIG. 5A illustrates a top view of one SOFC 1 of the stack 100. As shownin FIG. 5A, the SOFC 1 is positioned with the air side of theelectrolyte 5 pointing up. The cathode electrode 7 is located in themiddle portion of the electrolyte 5. The anode electrode 3 is located onthe bottom of the electrolyte and is not shown in FIG. 5A. Theelectrolyte 5 contains a fuel inlet opening 26A and a fuel outletopening 26B. The electrolyte also contains ring shaped regions 27A, 27Baround the openings 26A, 26B, respectively, and a peripheral region 28.The side cross sectional view of the stack shown in FIG. 3 is takenalong lines A-A in FIGS. 4A and 5A. The SOFC 1 is configured for a stackthat is internally manifolded for fuel and externally manifolded forair. Alternatively, the SOFC 1 may be configured for a stack which isinternally manifolded for both air and fuel. In this case, theelectrolyte would contain additional air inlet and outlet openings.

Openings 26A, 26B in electrolyte 5 are lined up with the respectiveopenings 16A, 16B in the interconnect 9 to form portions of the fuelinlet and outlet risers of the stack, as will be described in moredetail with respect to FIGS. 8A-8C below. A fuel riser is a series ofconnected openings through the interconnect 9 and/or through one or morelayers of the SOFC 1, such as the anode, cathode, and/or electrolytethrough which a fuel inlet or fuel outlet streams pass through the stack100.

FIGS. 6 and 7 illustrate a theory of electrolyte corrosion. In the priorart SOFC stack shown in FIGS. 6 and 7, the metal oxide (e.g., LSM) layer11 is located in contact with the ring seal 15. Without wishing to bebound by a particular theory, it is believed that manganese and/orcobalt from the manganese and/or cobalt containing metal oxide (e.g.,LSM, MCO, etc.) layer 11 leaches into and/or reacts with the glass seal15 and is then transported from the glass to the electrolyte. Themanganese and/or cobalt may be transported from the glass to theelectrolyte as manganese and/or cobalt atoms or ions or as a manganeseand/or cobalt containing compound, such as a manganese and/or cobaltrich silicate compound. For example, it is believed that manganese andcobalt react with the glass to form a (Si, Ba)(Mn,Co)O_(6±δ) mobilephase which is transported from the glass seal to the electrolyte. Themanganese and/or cobalt (e.g., as part of the mobile phase) at or in theelectrolyte 5 tends to collect at the grain boundaries of the zirconiabased electrolyte. This results in intergranular corrosion and pitswhich weaken the electrolyte grain boundaries, ultimately leading tocracks (e.g., hole 26A to hole 26B cracks) in the electrolyte 5. Withoutbeing bound by a particular theory, it is also possible that the fuel(e.g., natural gas, hydrogen and/or carbon monoxide) passing through thefuel inlet riser 36 may also react with the metal oxide layer 11 and/orthe glass seal 15 to create the mobile phase and to enhance manganeseand/or cobalt leaching from layer 11 into the seal 15, as shown in FIG.6.

The barrier(s) 12 a, 12 b of the embodiments of the invention reduce orprevent the interaction of the components of the LSM coating (or anotherMn or Co containing metal oxide coating) with the silica based glassseals and/or prevent the interaction of manganese contaminated silicabased glass seals with the electrolyte. Specifically, a barrier layerwhich preferably lacks any Mn and/or Co (or at least contains less than5 at % of Mn and/or Co) prevents Mn and/or Co diffusion from the metaloxide layer into the glass seal and/or prevents the Mn and/or Cocontaining mobile phase diffusion from the glass seal to theelectrolyte.

FIGS. 8A-8C illustrate a portion of a side cross sectional view alonglines B-B in FIGS. 4B and 5A of the fuel inlet riser portion of the SOFCstack 100 for three different embodiments. FIGS. 8A-8C illustrate howthe electrolyte 5 is protected from manganese and/or cobalt (and/or amanganese and/or cobalt containing compound, such as a manganese orcobalt rich silicate) diffusion from the electrically conductive metaloxide layer 11, such as a perovskite or spinel layer, for example an LSMand/or MCO layer on the interconnect 9, through a glass (or glassceramic) seal 15 into the electrolyte 5. In all three embodiments, oneor more manganese/cobalt diffusion barrier layers 12 a, 12 b are locatedbetween the electrolyte 5 and the conductive metal oxide layer 11 in theareas where the seal(s) 15 are located (e.g., in projections of areas27A and 27B on the electrolyte). In all three embodiments, the metaloxide layer 11 is located on the air surface of the interconnect 9. Themetal oxide layer directly physically contacts the cathode electrode 7on the electrolyte 5 of the adjacent SOFC in areas of the electrolyte 5that are not covered by the seal(s) 15. Thus, the barrier layer(s) arelocated around a fuel riser opening 36 between the seal 15 and at leastone of the metal oxide layer 11 and the electrolyte 5 but not over theSOFC cathode electrode 7 and preferably not over the ribs 10 and flowchannels 8 in the interconnect 9.

In FIG. 8A, the manganese diffusion barrier 12 a is located on the metaloxide layer 11 below the ring seal 15 and the electrolyte 5. In FIG. 8B,the manganese diffusion barrier 12 b is located on the ring seal 15between the seal and the electrolyte 5. Preferably, the barrier 12 b islocated on both the top and side(s) of the ring seal 15 to completelyseparate the seal from the electrolyte. In this embodiment, manganesethat diffuses into and reacts with the ring seal 15 material isprevented from reaching the electrolyte 5 by the diffusion barrier 12 b.In FIG. 8C, two manganese diffusion layers 12 a, 12 b are provided. Thefirst manganese diffusion barrier 12 a is located on the metal oxidelayer 11 below the ring seal 15 and the electrolyte 5 as provided in theembodiment illustrated in FIG. 8A. The second manganese diffusionbarrier 12 b is located on the top and sides of the ring seal 15 asprovided in the embodiment illustrate in FIG. 8B. The first and secondmanganese diffusion barrier layers 12 a, 12 b may be made from the sameor different materials. In this embodiment, the second manganesediffusion barrier 12 b provides extra diffusion prevention should anymanganese diffuse from the metal oxide layer 11 into the ring seal 15.While ring seals 15 are described above, it should be noted that thebarrier layer(s) 12 a, 12 b, may be located adjacent to any other sealson the air side.

Thus, the manganese diffusion barrier layer(s) 12 a and/or 12 b arelocated between the conductive metal oxide layer 11 and the electrolyte5. The diffusion barrier layer may be located between the conductivemetal oxide layer 11 and the glass ring seal 15 (barrier layer 12 a) orbetween the glass ring seal 15 and the electrolyte 5 (barrier layer 12b) or in both locations. Thus, even if manganese diffuses from theconductive metal oxide layer 11 into the glass ring seal 15, themanganese cannot further diffuse into the electrolyte layer 5.

FIGS. 8A-8C show one ring shaped glass (or glass ceramic) “ring” seal 15and adjacent barriers (e.g., 12 a) which is located on the air side ofeach interconnect 9 adjacent the fuel inlet opening 16A (i.e., a portionof the fuel inlet riser 36) extending through the interconnect 9 andmetal oxide layer 11. As shown in FIG. 8A, the interior opening 36A inthe seal 15 and barrier layer 12 a is located over the opening 16A inthe interconnect. The seal 15 also contacts an electrolyte 5 of anadjacent SOFC 1 in a region adjacent to a fuel inlet opening 26A in theelectrolyte, such that the opening 26A in the electrolyte, the interioropening 36A in the seal 15 and barrier 12 a, and the opening 16A in theinterconnect form a portion of the fuel inlet riser 36.

A second ring seal 15 and barrier layer(s) around the fuel outletopening 16B in the interconnect 9 are not shown for clarity. However, itshould be understood that a second ring shaped glass or glass ceramicseal 15 and barrier layer(s) 12 a and/or 12 b are located on the airside of each interconnect 9 over the fuel outlet opening 16B in theinterconnect 9 as shown in FIG. 4A. The opening 26B in the electrolyte,the interior opening in the second seal 15 and second barrier(s) and theopening 16B in the interconnect form a portion of the fuel outlet riser.

In an embodiment, the manganese diffusion barrier layer 12 a comprises aglass ceramic layer formed from a substantially glass barrier precursorlayer containing at least 90 wt. % glass (e.g., 90-100 wt. % glass, suchas around 99 to 100 wt. % amorphous glass and 0 to 1 wt. % crystallinephase) applied to a surface of interconnect 9 in the SOFC stack. In oneembodiment, the glass barrier precursor layer containing at least 90 wt.% glass comprises:

45-55 wt. % silica (SiO₂);

5-10 wt. % potassium oxide (K₂O);

2-5 wt. % calcium oxide (CaO);

2-5 wt. % barium oxide (BaO);

0-1 wt. % boron trioxide (B₂O₃);

15-25 wt. % alumina (Al₂O₃); and

20-30 wt. % zirconia (ZrO₂) on an oxide weight basis.

In one preferred embodiment, the glass barrier precursor layercomprises:

44.6 wt. % silica;

6.3 wt. % potassium oxide;

2.4 wt. % calcium oxide;

2.4 wt. % barium oxide;

19.1 wt. % alumina;

0.1 wt. % boron trioxide; and

25.1 wt. % zirconia on an oxide weight basis.

The use of a glass powder to make a glass barrier precursor layerfollowed by partially crystallizing the precursor layer to form a glassceramic barrier layer 12 a may improve properties of barrier layer 12 acompared to depositing a glass ceramic barrier layer directly over theinterconnect. For example, the efficacy of barrier layer 12 a inpreventing corrosion due to manganese and/or cobalt diffusion may beincreased because of the low porosity/high density of the initial glasslayer. In contrast, an as-deposited glass ceramic layer may have ahigher porosity/lower density compared to a glass layer which ispartially crystallized after deposition over the metal oxide contactlayer 11 on the interconnect 9. Additionally, the as-deposited glassbarrier precursor layer may exhibit superior adhesion to the metal oxidecontact layer 11 on the interconnect 9 as compared to an as-depositedglass ceramic layer. Thus, depositing a glass barrier precursor layer(e.g., in the form of a powder or powder in a binder) followed bypartially crystallizing it to form a glass ceramic barrier layer resultsin better barrier layer properties than directly depositing a glassceramic layer or powder.

A method of forming a planar, electrolyte supported SOFC stack shown inFIGS. 3 and 8A-8C includes forming SOFCs 1 and interconnects 9 andalternating these SOFCs and interconnects in a stack 100.

The SOFC 1 is formed by forming the openings 26A, 26B in the electrolyte5. Then, a cathode electrode 7 is formed on the first side of theelectrolyte and an anode electrode 3 is formed on the second side of theelectrolyte. The electrodes may be formed by screen printing or othersuitable deposition methods. At least one of the electrolyte, cathodeelectrode and the anode electrode are then fired or sintered. One ormore firing or sintering steps may be conducted. For example, oneelectrolyte firing step may be conducted after the opening 26A, 26Bformation (e.g., by hole punching), another firing step after cathodedeposition and a third firing step after the anode deposition. The anodeand cathode deposition may be performed in either order. The threefiring steps may be combined into two firing steps or into a singlefiring step after both electrodes are deposited.

If desired, the second barrier layer 12 b shown in FIGS. 8B and 8C maybe formed on the cathode 7 side of the electrolyte 5 by depositing thebarrier powder (optionally with a binder) around the fuel riser openingsfollowed by burning out the binder and sintering the powder. The barrierlayer 12 b may be deposited and sintered before, after, or at the sametime as the cathode electrode 7.

In one embodiment, the coated interconnect 9 is formed as follows. Asillustrated in FIG. 8A, the manganese diffusion barrier layer 12 a maybe formed on the electrically conductive contact layer 11, such as ametal oxide layer (e.g., LSM and/or MCO layer) which is deposited on theair surface of interconnect 9.

In one non-limiting example, the manganese diffusion barrier layer 12 amay be formed as follows. Starting material powders, such as alkali oralkali earth carbonates (e.g., potassium carbonate, calcium carbonate,and/or barium carbonate) and/or metal or metalloid oxides (e.g.,aluminum oxide, boron oxide, silica, and/or zirconia) are combined in apowder mixture. The powder mixture is then melted at a temperature of atleast 1400° C. (e.g., 1500-1550° C.) to form a precursor melt. The meltis then quickly cooled (e.g., quenched) to form a glass body that is atleast 90 wt. % glass (e.g., 90 to 100 wt. % glass, such as 99-100%glass).

The glass body is then crushed into a glass powder. The glass powder (orglass powder in a binder) is then applied over interconnect 9. Forexample, the glass powder may be applied on the metal oxide layer 11located on the air side of the interconnect 9 in the fuel riser regionsadjacent to a fuel inlet and/or fuel outlet opening 16A, 16B (e.g., thefuel riser opening). The applied glass powder forms a glass barrierprecursor layer over regions on which the ring seal will be provided.Thus, the first barrier layer 12 a shown in FIGS. 8A and 8B may bedeposited on the interconnect as a glass powder barrier precursor layercontaining an optional binder around the fuel riser openings 16A, 16B,followed by burning out the binder (if present).

Optionally, the glass powder of the barrier precursor layer may besintered or densified before the seals 15 are formed on the barrierprecursor layer and before the interconnect is placed into the stack.Alternatively, the barrier precursor layer may be sintered together withthe seals after the interconnect 9 is provided into the stack 100.

The ring seal(s) 15 are then formed on the surface of the manganesediffusion barrier precursor layer. The glass or glass ceramic ring seal15 may be applied over the glass barrier precursor layer in regionsadjacent to a fuel inlet 16A and/or fuel outlet 16B opening in theinterconnect 9.

The interconnect 9 is then placed into a SOFC stack 100 containing thefuel cells 1 and other interconnects 9. In regions of the metal oxidelayer 11 that are not covered by the ring seal 15 (not shown), the glassbarrier precursor layer may directly contact the cathode electrode 7 onthe electrolyte 5 of an adjacent SOFC 1 in the stack 100.

The SOFC stack is then sintered (i.e., heated at a temperature of atleast 900° C., such as 900-950° C.) for 1-10 hours (e.g., 2-5 hours) tomelt and set the ring seal(s) 15 and to convert the glass barrierprecursor layer(s) to glass ceramic barrier layer(s) 12 a.

During the sintering, the glass barrier precursor layers in the stackare partially crystallized to form glass ceramic manganese/cobaltbarrier layers 12 a containing crystalline phases distributed in aglassy (e.g., amorphous) matrix phase. Without wishing to be bound by aparticular theory, it is believed that zirconium silicate (ZrSiO₄)crystals (i.e., crystalline phase), which provide the corrosionresistance and manganese/cobalt barrier properties, may precipitate(e.g., nucleate and grow) in the glassy matrix. Other “filler” crystals(i.e., crystalline phases) that allow stack sintering at relatively lowtemperatures (e.g., below 1000° C.) may also precipitate in the glassymatrix, such as barium aluminum silicate (BaAl₂Si₂O₈) and potassiumfeldspar (KAlSi₃O₈) crystals. All of these materials may crystallize atdifferent temperatures from one another. For example, when the sinteringtemperature reaches about 820° C., barium aluminum silicate crystals mayprecipitate first, followed by zirconium silicate crystals, and then bypotassium feldspar crystals. The resulting glass ceramic layer (i.e.,zirconium silicate crystals and filler crystals in the remaining glassymatrix) form the manganese/cobalt diffusion barrier layer 12 a. The SOFCstack operates in temperatures ranging from 750 to 1,000° C. It isbelieved that after around 2000 hours of operation of the SOFC stack,further filler crystals, such as alumina crystals, may form (e.g.,nucleate and grow) in the barrier layer 12 a.

The manganese/cobalt diffusion barrier layer 12 a composition depends onthe composition of the glass powder that becomes the glass barrierprecursor layer on interconnect 9. In an embodiment, the glass powder inthe glass barrier precursor layer may have a composition ofSiO₂-(M1)₂O-(M2)O-(M3)₂O₃-(M4)O₂. M1 may comprise an alkali metal, suchas at least 80 wt. % potassium, such as 80-100% potassium. M2 maycomprise an alkali earth metal, such as at least 80 wt. % barium and/orcalcium, such as 80-100 wt. % barium and calcium (e.g., 2:1 to 1:2, suchas 1:1 weight ratio of Ba to Ca). M3 may comprise a Group 13 (i.e.,Group IIIA) element of the Periodic Table of Elements, such as at least80 wt. % boron and aluminum, such as 90-100 wt. % Al and 0-10 wt. %boron. M4 may comprise a transition metal, such as at least 80 wt. %zirconium, such as 80-100 wt. % zirconium. For example, the glassbarrier precursor layer composition may include silica, potassium oxide(K₂O), barium oxide (BaO), calcium oxide (CaO), alumina, zirconia, andoptionally boron trioxide (B₂O₃).

In particular, the range of silica content may be 45-55 wt. % in theglass powder. If there is less than 45 wt. % silica, then there may betoo little silica to form a substantially glass body by quenching theprecursor melt. In other words, an undesirable amount of crystals may bepresent in the glass body quenched from the precursor melt. If there isgreater than 55 wt. % silica, then there may be too much silica basedglassy matrix phase and insufficient amount of the zirconium silicatecrystals in the barrier layer 12 a, which may degrade the barriercorrosion prevention properties.

The range of K₂O content may be 5-10 wt. % in the glass powder. If thereis less than 5 wt. % K₂O, then there may too little potassium to createa sufficient amount of potassium feldspar crystals. If there is morethan 10 wt. % K₂O, then the resulting barrier layer 12 a composition maycontain too much of the glassy matrix phase after sintering.

The range of CaO and BaO content may each be 2-5 wt. % in the glasspowder (e.g., 4-10 wt. % alkali earth oxide). If there is less than 2wt. % of CaO and of BaO, then there may be insufficient calcium tocreate a sufficient amount of potassium feldspar crystals andinsufficient barium to create a sufficient amount of barium aluminumsilicate crystals, respectively. However, if there is more than 5 wt. %of either CaO or BaO (i.e., more than 10 wt. % of alkali earth oxide),then the coefficient of thermal expansion (CTE) of the barrier layer 12a may become too different from the CTE of the interconnect 9 (e.g., achromium (Cr) and 4 to 6 wt. % iron (Fe) alloy interconnect) and causean undesirable CTE mismatch between the interconnect and the barrierlayer.

The range of zirconia content may be 20-30 wt. % in the glass powder. Ifthere is less than 20 wt. % zirconia, then there may be insufficientzirconium to make a sufficient amount of the corrosion-resistant ZrSiO₄crystals. If there is more than 30 wt. % zirconia, then the precursorpowder composition becomes difficult to melt completely into theprecursor melt at a reasonable temperature (e.g., 1550° C. or below) andit becomes difficult to form the glass body from the melt withoutincluding zirconia or zirconium silicate crystals.

The range of boron trioxide content may be 0-1 wt. % in the glasspowder. Preferably, as little as possible of boron trioxide is used,since it negatively affects the barrier layer 12. Thus, it may beomitted entirely. However, boron trioxide is a sintering aid and may beused in a small amount of 1 wt. % or less to enhance melting andsintering at a reasonable temperature.

The range of alumina content may be 15-25 wt. % in the glass powder.Alumina is a balancing component which provides the balance of aluminumneeded (without excess) to form barium aluminum silicate crystals andpotassium feldspar crystals. Thus, the 15-25 wt. % range provides asufficient amount of aluminum to form these crystals.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

What is claimed is:
 1. A method of forming a diffusion barrier layer,comprising: providing an interconnect for a fuel cell stack, wherein theinterconnect contains an electrically conductive contact layer locatedon an air surface of the interconnect, and wherein the electricallyconductive contact layer contains at least one of Co and Mn; forming abarrier precursor layer comprising at least 90 wt. % glass over theelectrically conductive contact layer; and heating the barrier precursorlayer to precipitate crystals in the barrier precursor layer to convertthe barrier precursor layer to a glass ceramic barrier layer.
 2. Themethod of claim 1, wherein the electrically conductive contact layercomprises a metal oxide contact layer comprising at least of Mn and Co.3. The method of claim 1, wherein: the metal oxide contact layercomprises lanthanum strontium manganite (LSM), manganese cobalt oxidespinel (MCO) or a mixture thereof; the interconnect comprises achromium-iron alloy interconnect having gas flow channels and ribs; andthe barrier precursor layer comprises 99-100 wt % glass.
 4. The methodof claim 2, further comprising forming a glass or glass ceramic seal inat least one region over the barrier precursor layer.
 5. The method ofclaim 4, wherein the at least one region comprises a fuel riser regionaround a fuel riser opening through the interconnect, and the sealcomprises a ring seal formed around the fuel riser opening.
 6. Themethod of claim 4, further comprising placing the interconnectcontaining the metal oxide contact layer, the glass ceramic barrierlayer and the seal into a solid oxide fuel cell (SOFC) stack comprisinga plurality of solid oxide fuel cells and a plurality of additionalinterconnects prior to the step of heating.
 7. The method of claim 6,wherein the step of heating comprises sintering the SOFC stack at atemperature of at least 900° C. for 1-10 hours.
 8. The method of claim7, wherein sintering the SOFC stack is performed at a temperature of900-950° C. for 2-5 hours.
 9. The method of claim 7, wherein the glassceramic barrier layer prevents or reduces at least one of Mn and Codiffusion from the electrically conductive contact layer to at least oneof the seal and an adjacent SOFC in the SOFC stack.
 10. The method ofclaim 1, further comprising: melting a mixture of starting materialpowders at a temperature greater than 1400° C.; quenching the meltedmixture to form a glass body; and crushing the glass body to form aglass powder mixture of at least 90 wt. % glass.
 11. The method of claim10, wherein the step of forming the barrier precursor layer comprisesdepositing the glass powder mixture on the electrically conductivecontact layer.
 12. The method of claim 11, wherein the glass powdermixture has a composition of SiO₂-(M1)₂O-(M2)O-(M3)₂O₃-(M4)O₂, where M1comprises at least 80 wt. % potassium, M2 comprises at least 80 wt. %barium and calcium, M3 comprises at least 80 wt. % aluminum, and M4comprises at least 80% zirconium.
 13. The method of claim 12, whereinthe mixture of starting material powders comprises potassium carbonate(K₂CO₃), calcium carbonate (CaCO₃), barium carbonate (BaCO₃), alumina(Al₂O₃), boron trioxide (B₂O₃), silica (SiO₂), and zirconia (ZrO₂). 14.The method of claim 13, wherein the glass powder mixture comprises:45-55 wt. % silica (SiO₂); 5-10 wt. % potassium oxide (K₂O); 2-5 wt. %calcium oxide (CaO); 2-5 wt. % barium oxide (BaO); 0-1 wt. % borontrioxide (B₂O₃); 15-25 wt. % alumina (Al₂O₃); and 20-30 wt. % zirconia(ZrO₂) on an oxide weight basis.
 15. The method of claim 14, wherein theglass powder mixture comprises: 44.6 wt. % silica; 6.3 wt. % potassiumoxide; 2.4 wt. % calcium oxide; 2.4 wt. % barium oxide; 19.1 wt. %alumina; 0.1 wt. % boron trioxide; and 25.1 wt. % zirconia on an oxideweight basis.
 16. The method of claim 1, wherein the crystals in theglass ceramic barrier layer comprise zirconium silicate (ZrSiO₄)crystals located in a glassy phase, and wherein the zirconium silicatecrystals form a barrier to at least one of manganese and cobaltdiffusion from the electrically conductive contact layer through theglass ceramic barrier layer.
 17. The method of claim 16, wherein thecrystals further comprise barium aluminum silicate (BaAlSiO₄) crystalsand potassium feldspar (KAlSi₃O₈) crystals located in the glassy matrix.18. The method of claim 17, further comprising precipitating additionalalumina crystals in the glass ceramic barrier layer by operating theSOFC stack for at least 2000 hours.
 19. The method of claim 1, furthercomprising: forming a glass or glass ceramic ring seal over the barrierprecursor layer in at least one fuel riser region around a fuel riseropening through the interconnect; and placing the interconnectcontaining the metal oxide contact layer, the glass ceramic barrierlayer and the seal into a solid oxide fuel cell (SOFC) stack comprisinga plurality of solid oxide fuel cells and a plurality of additionalinterconnects prior to the step of heating; wherein: the electricallyconductive contact layer comprises lanthanum strontium manganite (LSM),manganese cobalt oxide spinel (MCO) or a mixture thereof; theinterconnect comprises a chromium-iron alloy interconnect having gasflow channels and ribs; the barrier precursor layer comprises 99-100 wt% glass; the step of heating comprises sintering the SOFC stack at atemperature of at least 900° C. for 1-10 hours; the glass ceramicbarrier layer prevents or reduces at least one of Mn and Co diffusionfrom the electrically conductive contact layer to at least one of theseal and an adjacent SOFC in the SOFC stack; the crystals in the glassceramic barrier layer comprise zirconium silicate (ZrSiO₄) crystals,barium aluminum silicate (BaAlSiO₄) crystals and potassium feldspar(KAlSi₃O₈) crystals located in a glassy matrix; and the barrierprecursor layer comprises: 45-55 wt. % silica (SiO₂); 5-10 wt. %potassium oxide (K₂O); 2-5 wt. % calcium oxide (CaO); 2-5 wt. % bariumoxide (BaO); 0-1 wt. % boron trioxide (B₂O₃); 15-25 wt. % alumina(Al₂O₃); and 20-30 wt. % zirconia (ZrO₂) on an oxide weight basis.
 20. Asolid oxide fuel cell (SOFC) stack, comprising: a plurality of SOFCs;and a plurality of interconnects, wherein each of the plurality of theinterconnects is located between two adjacent SOFCs; wherein each of theplurality of the interconnect comprises: an electrically conductivecontact layer located on an air surface of the interconnect, theelectrically conductive contact layer containing at least one of Co andMn; and a glass ceramic barrier layer located over the electricallyconductive contact layer, the glass ceramic barrier layer comprisingzirconium silicate (ZrSiO₄) crystals, barium aluminum silicate(BaAlSiO₄) crystals and potassium feldspar (KAlSi₃O₈) crystals locatedin a glassy matrix.
 21. The SOFC stack of claim 20, wherein the SOFCstack further comprises a glass or glass ceramic seal located betweeneach glass ceramic barrier layer and a cathode electrode of eachadjacent SOFC.
 22. The SOFC stack of claim 21, wherein the sealcomprises a ring seal located over the glass ceramic barrier layer in atleast one fuel riser region around a fuel riser opening through theinterconnect.
 23. The SOFC stack of claim 22, wherein: the electricallyconductive contact layer comprises lanthanum strontium manganite (LSM),manganese cobalt oxide spinel (MCO) or a mixture thereof; theinterconnect comprises a chromium-iron alloy interconnect having gasflow channels and ribs; the glass ceramic barrier layer prevents orreduces at least one of Mn and Co diffusion from the electricallyconductive contact layer to at least one of the seal and an adjacentSOFC in the SOFC stack; and the glass ceramic barrier layer comprises:45-55 wt. % silica (SiO₂); 5-10 wt. % potassium oxide (K₂O); 2-5 wt. %calcium oxide (CaO); 2-5 wt. % barium oxide (BaO); 0-1 wt. % borontrioxide (B₂O₃); 15-25 wt. % alumina (Al₂O₃); and 20-30 wt. % zirconia(ZrO₂) on an oxide weight basis.
 24. The SOFC stack of claim 23,wherein: the glass ceramic barrier layer further comprises aluminacrystals located in the glassy matrix; and the glass ceramic barrierlayer comprises: 44.6 wt. % silica; 6.3 wt. % potassium oxide; 2.4 wt. %calcium oxide; 2.4 wt. % barium oxide; 19.1 wt. % alumina; 0.1 wt. %boron trioxide; and 25.1 wt. % zirconia on an oxide weight basis.
 25. Aninterconnect for a fuel cell stack, comprising an interconnect bodyhaving an air surface having air flow channels and ribs and a fuelsurface having fuel flow channels and ribs; an electrically conductivecontact layer located on the air surface of the interconnect, theelectrically conductive contact layer containing at least one of Co andMn; and a first layer located over the electrically conductive contactlayer, the first layer comprising: 45-55 wt. % silica (SiO₂); 5-10 wt. %potassium oxide (K₂O); 2-5 wt. % calcium oxide (CaO); 2-5 wt. % bariumoxide (BaO); 0-1 wt. % boron trioxide (B₂O₃); 15-25 wt. % alumina(Al₂O₃); and 20-30 wt. % zirconia (ZrO₂) on an oxide weight basis. 26.The interconnect of claim 25, wherein the first layer comprises a glassbarrier precursor layer.
 27. The interconnect of claim 25, wherein thefirst layer comprises a glass ceramic barrier layer.
 28. Theinterconnect of claim 27, further comprising a glass or glass ceramicseal located over the first layer.
 29. The interconnect of claim 28,wherein: the glass ceramic barrier layer comprises zirconium silicate(ZrSiO₄) crystals, barium aluminum silicate (BaAlSiO₄) crystals andpotassium feldspar (KAlSi₃O₈) crystals located in a glassy matrix; theseal comprises a ring seal located over the glass ceramic barrier layerin at least one fuel riser region around a fuel riser opening throughthe interconnect; the electrically conductive contact layer compriseslanthanum strontium manganite (LSM), manganese cobalt oxide spinel (MCO)or a mixture thereof; the interconnect comprises a chromium-iron alloyinterconnect; and the glass ceramic barrier layer prevents or reduces atleast one of Mn and Co diffusion from the electrically conductivecontact layer to the seal.
 30. The interconnect of claim 29, wherein:the glass ceramic barrier layer further comprises alumina crystalslocated in the glassy matrix; and the glass ceramic barrier layercomprises: 44.6 wt. % silica; 6.3 wt. % potassium oxide; 2.4 wt. %calcium oxide; 2.4 wt. % barium oxide; 19.1 wt. % alumina; 0.1 wt. %boron trioxide; and 25.1 wt. % zirconia on an oxide weight basis.